For those who need to wear a prosthetic limb, it is a fundamental desire to walk naturally in the same way as healthy persons do. Particularly, it is a desire, almost like a dream, for them to walk up and down steps and/or walk down a downhill smoothly by moving the left and right feet alternately. A flexible knee braking function, namely, yielding function is a function for realizing the latter desire of those who need to wear a prosthetic limb. Owing to this yielding function, the knee joint is slowly changed in bending angle when the wearer's load is imposed on the prosthetic limb including a knee joint. Therefore, the wearer of the prosthetic limb having such a yielding function can smoothly walk down steps and a downhill by placing his/her weight on the prosthetic limb with confidence and moving the left and right limbs alternately.
From another view point, it can be said that the yielding technique is a technique for flexibly locking the prosthetic limb in a bendable condition compared with a technique for completely locking the prosthetic limb (bending motion of the prosthetic limb). The technique for completely locking the prosthetic limb is a technique for producing a braking force by tightening a knee axis by utilizing a mechanical friction force, as shown, for example, in U.S. Pat. No. 3,863,274 (corresponding to Japanese Patent Publication No. Sho 52-46432). In contrast, the technique for flexibly locking the prosthetic limb is a technique for producing a braking force by utilizing a flow resistance generated when a working oil or fluid passes through a throttle. Therefore, the flexible lock mechanism comprises a hydraulic brake circuit including several hydraulic equipments, besides the throttle. One of the hydraulic equipments included in the hydraulic brake circuit is means for defining two chambers into which or from which the working fluid flows. Known as the means are a piston type including a reciprocally movable piston and a rotary type including a turnable vane. U.S. Pat. No. 5,704,945 (corresponding to Japanese Patent Application Laid-Open No. H08-317944), U.S. Pat. No. 2,667,644, etc. disclose the rotary type, and U.S. Pat. No. 2,530,286, U.S. Pat. No. 2,568,053, etc. disclose the piston type.